linux-kernel - Re: [RFC PATCH v2] lib: scatterlist: add sg splitting function

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Date:	Mon, 3 Aug 2015 16:19:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Jens Axboe <axboe@...nel.dk>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [RFC PATCH v2] lib: scatterlist: add sg splitting function

On Sat,  1 Aug 2015 15:17:13 +0200 Robert Jarzmik <robert.jarzmik@...e.fr> wrote:

> Sometimes a scatter-gather has to be split into several chunks, or sub scatter
> lists. This happens for example if a scatter list will be handled by multiple
> DMA channels, each one filling a part of it.
> 
> A concrete example comes with the media V4L2 API, where the scatter list is
> allocated from userspace to hold an image, regardless of the knowledge of how
> many DMAs will fill it :
>  - in a simple RGB565 case, one DMA will pump data from the camera ISP to memory
>  - in the trickier YUV422 case, 3 DMAs will pump data from the camera ISP pipes,
>    one for pipe Y, one for pipe U and one for pipe V
> 
> For these cases, it is necessary to split the original scatter list into
> multiple scatter lists, which is the purpose of this patch.
> 
> ...
>
>  include/linux/scatterlist.h |   5 ++
>  lib/scatterlist.c           | 189 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 194 insertions(+)

It's quite a bit of code for a fairly specialised thing.  How ugly
would it be to put this in a new .c file and have subsystems select it
in Kconfig?

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